Shell Structures

Shells are characteristic structure forms in nature. The most well-known are snail or mussel shells. The shells of mussels (examples in Fig. 5.25) in particular have inspired building forms, and not only as decorative elements. Even in antiquity one had attempted to realize their wide-stretched, often thin-walled forms, but analogous forms were only enabled with the building materials of modern era, pre-stressed concrete above all.

Architects such as Le Ricolais attempted early on to observe, understand, and abstract such shells as building–structural entities.

5.5.1   Mussel Shells → “Isoflex”

In adoption of the rib structure and other structural idiosyncrasies of the shells of the large scallop, Pecten jacobaeus, Le Ricolais conceptualized a structural system

Fig. 5.25  Mussel shells: a the pilgrim’s scallop Pecten jacobaeus, width 10 cm and b giant clam, Tridacna spec, width 120 cm. (Adapted from Coineau, Kresling 1987)

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consisting of perpendicularly crossing corrugated panels, which he described as

“isoflex” (Fig. 5.26).

One can bend a layer of this structure into a tube form as well and anchor it in a given round tube, thereby producing lightweight structural tubes with significant strength and rigidity.

5.5.2   Shells Similar to Tridacna → Shell Structures

Figure 5.27 shows some examples as to how the analyses of the shells of the gi-ant clam have influced the conception of shell-like, long-spanning structures. A restaurant in Xochimilco, Mexico (Fig. 5.27a), represents a hyperbolic parabaloid with a shell thickness measuring only 15 mm. It is a geometric structure that is self-supporting. The interpretation of this form is more clearly visible with the market hall in Royan, France, built by Simon et al. in 1955. The roof with its radial wave-forms has a span distance of 52.4 m. Even broader is the wavy roof of the national circus in Bucharest, Romania, by Porumbescu et al. in 1960 (Fig. 5.27c). It spans a distance of 66.6 m.

Fig. 5.26  Pilgrim’s scallop and “isoflex” abstraction.

(Adapted from Bull. Ing. Civ.

de France, bottom drawing;

Kresling from Nachtigall 1987, presentation par A.

Bougrain-Dubourg)

With all of these shell structures a direct translation was not the ultimate goal, though it is known that the architects were inspired by the elegance of natural shell forms and played with these difficult to realize building forms.

Heinz Isler, as a modern representative, attempted long-spanning shell structures as well. He is major fan of gardens and had spent a lot of time in nature to become imprinted with the natural, botanical, zoological forms. His structures, which ap-pear in Switzerland, for example as a roof cover for highway gas station, are excep-tionally thin in comparison to their spanning length.

Shells consisting of concrete must be formed so that the supporting network of prestressed concrete does not deviate by more than a few millimeters from the

Figure 5.27  Tridacna-like shell structures, a restaurant, Xochimilco, Mexico, hyperbolic parabo-loid, shell thickness merely 1.5 cm (!), b market hall Royan, France. Simon et al. 1955. Radially corrugated roof, span distance 52.4 m, c building for the national circus, Bucharest, Romania.

Porumbescu et al. 1960. Radially corrugated roof, span distance 66.6 m. a from Blaser (Ed.) 1985 and b, c from Lebedev 1983)

5.5 Shell Structures 161

plane of the self-supporting form. In the case of Heinz Isler, the form is even found by hanging chains; the coordinates of the perpendicularly cutting catenary curves were marked. Such a shell is—performed in the thought experiment by “welding”

the hanging chains together at their meeting points and “flipping it over”—self-sup-porting. The structure can be entirely formed in this manner, so that it only develops compression forces at the supports. Of course, the function ultimately influences the form and therefore the design of the layout.

Figure 5.28 shows, extracted from the illustration by Patzelt 1974 and lightly supplemented, sections of shell structures with specifications of their diameters, shell thicknesses, and the ratio between the two measurements. Accordingly, a mar-ket hall in Algeria from 1955 with a shell thickness corresponds to about 1/1000 of its diameter and the 60 m spanning sport arena in Rome from 1956 (Nervi) with a relative shell thickness of 1/2400. The impressive building achievement from an-tiquity that is the Pantheon, built in the second century, must also appear here; its relative shell thickness is comparatively high with 1/44.

The proverbial chicken egg comes up short in comparison as well; its thickness ratio amounts to 1/112. The porous lime skeleton of the eggshell material is of course not an ideal building material. However, the form is so “refined” that it can withstand high compression forces. One cannot break an undamaged chicken egg

Fig. 5.28  Examples of early shell structures and their dimensions in comparison to a chicken egg: a market hall, Algeria, 1955, b palazzetto dello sport, Rome, 1956, c arena, Canada, 1958, d fac-tory building, Jena 1923, and e shell of the chicken egg, F Pantheon, Rome, second cen-tury. (Adapted from Patzelt 1974)

between the thumb and index finger. The “egg-shaped” envelope of the first nuclear reactor in Germany, the famous “atomic egg of Garching,” was according to the architect not developed with the egg in mind. The form was the result of consider-ations of functionality, namely, how can the interior space with a given footprint be spanned with the lowest possible material expenditure. Due to this basic consider-ation an approximately “egg-shaped” shell form developed itself, which had been immediately classified colloquially as the “atomic egg.” One can establish an anal-ogy a posteriori, a similarity of form, as it often results in comparison of biological and technological structures.

The dome of St. Peter’s in Rome originates from Michelangelo among others, built in 1561 (Fig. 5.29). “The eggshell is one of the few living structures that inspired major builders, as we know from historic dome structures” (F. Otto). The dome is formed as a double-layered shell, with the exterior layer and the interior ceiling formed differently. As the indication lines show, the dome is actually nei-ther egg-shaped nor parabolic. Ultimately it was not formed as a catenary (inverted chain line). Despite extreme similarity, the egg is in this case not the exact inspira-tion and apparently not the catenary model as well.

5.5.3   Sea Urchin Shells → Inspiration for Structure

Sea urchins form very peculiar housings that must be structurally stable not only in their finished form but also during their “construction.” Sea urchin shells and their qualities have already been illuminated in Sect. 2.1.5 under “Panel Structures.”

Their shape provided inspiration for the concept for the ice sport arena in Erfurt

Fig. 5.29  Double shell of St.

Peter’s, Rome, Michelangelo et al., from 1561, in com-parison with parabolic and catenary curves. (Adapted from Patzelt 1974, edited)